1. Field of the Invention
The present invention relates to a polymer dispersed liquid crystal material, a polymer dispersed liquid crystal display device and a method of manufacturing the same.
2. Description of the Prior Art
Liquid crystal display devices are excellent, as compared with other types of display, in terms of low power consumption, flatness and therefore portability so that many portable displays, projectors and the display devices of other electric appliance are formed therefrom. Among them, projection-type liquid crystal displays (projection-type LCDs) are particularly advantageous as compared with CRT (cathode ray tube) to realize wide screens with relatively small and light weight projector propers. Because of this, the study of application for the LCDs to projection televisions have been enhanced.
The projection-type LCDs are composed of a pair of polarizing plates between which is located a liquid crystal panel utilizing a twisted nematic liquid crystal material in order to modulate projected light in terms of intensity. However, a polarizing plate absorbs at least a half of incident light quantity passing therethrough so that the total transmittance is substantially decreased. The displayed image is therefore dimmed when the display panel is utilized in a well-lit room.
From this view point, there have been needs for a liquid crystal display panel having a high transmittance. The use of polymer dispersed liquid crystal materials was then proposed to realize a liquid crystal display panel having a high transmittance.
In such a polymer dispersed liquid crystal panel, the liquid crystal device is constructed as illustrated in FIGS. 1A and 1B.
The opposed inside surfaces of a first transparent substrate 101 and a second transparent substrate 102 are coated with transparent electrodes 103 and 104 in FIGS. 1A and 2A. Polymer dispersed liquid crystal layers 105 and 106 are disposed between these transparent electrodes. Namely, the polymer dispersed liquid crystal layer 105 as illustrated in FIG. 1A is composed of a polymer matrix 105a in which are embedded a number of micron-sized droplets of a nematic liquid crystal material 105b. On the other hand, the polymer dispersed liquid crystal layer 106 as illustrated in FIG. 2A is composed of a nematic liquid crystal matrix 106b in which are embedded a number of micron-sized particles of a polymer 106a. Another type of polymer dispersed liquid crystal panel is composed of a nematic liquid crystal material dispersed in a polymer matrix in the form of a network structure as described in [1] E. Simada, T. Uchida, JAPAN DISPLAY '92, pp. 699-702.
As illustrated in FIGS. 1A and 2A, the molecules of the nematic liquid crystal 105b and 106b are aligned at random in the absence of an electric field applied between the transparent electrodes 103 and 104 so that light rays passing therethrough are substantially scattered. In this case, there is a differential index of refraction at the boundary between the nematic liquid crystal 105b and 106b and the polymer matrix 105a and 106a to enhance the effect of scattering light rays.
Contrary to this, as shown in FIGS. 1B and 2B, when an electric field applied between the transparent electrodes 103 and 104, the molecules of the nematic liquid crystal 105b and 106b are directed normal to the transparent electrodes 103 and 104 in alignment with the electric field as illustrated in FIGS. 1B and 2B. The light rays are therefore passed therethrough without scattering since there is substantially no variation in the index of refraction either on the nematic liquid crystal 105b and 106b and the polymer matrices 105a and 106a. A polarizing plate is therefore not required in this structure so that the efficiency of making use of the incident light is substantially improved.
When the light transmittance of a polymer dispersed liquid crystal panel is plotted while the voltage thereacross as the abscissa is increased from a higher level to a lower level and decreased from the lower level to the higher level, there is observed a hysteresis effect since there are differences between the curves of the hysteresis loop illustrated when the voltage is increased and when the voltage is decreased as reported in [2] PAUL S. DRZAIC, LIQUID CRYSTALS, 1988, VOL.3, NO.11, pp. 1543-1559. The hysteresis causes afterimages to linger on and be burned into the screen of the panel.
Accordingly, when the polymer dispersed liquid crystal panel as described above is driven in the same manner as usual twisted nematic liquid crystal display, it is impossible to realize grayscale images and full color images.
In order to reduce the width of the hysteresis, it is proposed to add fluorinated acrylate monomer to a polymer dispersed liquid crystal material in [3] N. Yamada et al., JAPAN DISPLAY 1992, pp.695-698 and to modify the proportion of acrylate monomer in a polymer dispersed liquid crystal layer in [4] J. De Baets et al., EURO DISPLAY 1993, pp.117-120.
However, the conventional techniques including the above two examples for reducing the width of the hysteresis can not be applied to polymer dispersed liquid crystal panels utilizing usual liquid crystal layers and usual polymer matrices.